Air separation plants generally include adsorbers or adsorber vessels to remove contaminants such as carbon dioxide, water, hydrocarbons, etc. from the air before the air enters the air distillation plants. The classical adsorber design uses cylindrical vessels where the minimum cross section of the cylindrical adsorber vessel is determined by the maximum permissible flow rate of a rising gaseous stream (e.g., air). The cylindrical absorber vessel may be positioned vertically or horizontally based on the requirements for the vessel. For both vertical and horizontal adsorber configurations the maximum permissible flow rate of the rising gaseous stream is set by the fluidization (i.e., the velocity at which the feed gas may lift the adsorbent). This is especially true at the bed surface, and in the case of a horizontal vessel configuration, near the bed surface near the inner vessel wall.
As illustrated in FIG. 1A, traditional horizontal adsorbers 100 comprise a cylindrical vessel wall 102 that contains the adsorbent 104. The adsorbent 104 sits on top of an adsorbent bed support 116. The adsorbent has top surface layer of adsorbent 114 that extends along the width and length of the adsorber vessel 102. As illustrated in FIG. 1B, small “dunes” 108 begin to form on the surface layer of adsorbent 114 close to the vessel wall inner surface 106 as a result of the forces placed on the adsorbent by the rising gaseous streams through the adsorbent.
In traditional adsorber designs, and as illustrated in FIGS. 1A and 1B, the angle of the vessel wall inner surface 106 and the shape of the adsorbent 104, causes small spaces or voids 120 to be created near the vessel wall inner surface 106 where the adsorbent 104 is unable to fill. Rising gaseous streams looking for the path of least resistance, tend to flow through these small spaces or voids 120 along the vessel wall inner surface 106 rather than flowing through the more resistant sections filled with the adsorbent 104. Because the adsorbent 104 near the vessel wall inner surface 106 is not “caged” as well as the adsorbent 104 located near the center of the adsorber 100, and because the forces placed on the adsorbent 104 proximate to both the vessel wall inner surface 106 and the small spaces or voids 120 from the rising gaseous streams is more significant than the countering weight of the adsorbent 104, these small dunes 108 tend to form. As the adsorber is cycled from use to regeneration and back to use, small areas of adsorbent proximate to the dunes 108 and near the vessel wall inner surface 106 begin to move thereby causing fluidization. Fluidization is undesirable because it leads to adsorbent mixing, attrition of adsorbent, and dusting. As a result of the mixing, attrition and dusting, adsorbers will experience high pressure drop and loss of performance because of reduction in adsorbent diameter and reduction in adsorbent active surface area. Consequently the raw gas passes through the void regions 120 and 122 leading to earlier contaminant breakthrough. Fluidization along the vessel wall inner surface is dependent on the superficial velocity as higher superficial velocity means higher upward force under the adsorbent bed. When this upward force exceeds the adsorbent downward weight force fluidization occurs. As illustrated in FIG. 1C, at the widest point of the horizontal vessel, the wall angle or the angle (α) between the tangent of the inner vessel wall (or plane A) and the horizontal plane B is approximately ninety (90°) degrees. At such point in the horizontal vessel, the flow is uniform. As the flow moves upward through the horizontal vessel, however, the wall angle or the angle (β) between the tangent of the inner vessel wall (or plane D) and the horizontal plane C decreases. The flow streamlines near the inner vessel wall start to converge while at the center of the horizontal vessel the streamlines remain uniform. This convergence of the streamlines at or near the vessel wall leads to higher flow velocities and hence earlier fluidization at or near the vessel wall. As the wall angle decreases the convergence of the streamlines further increases leading to higher velocities and fluidization in localized region 122.
To suppress fluidization, thereby limiting loss of performance of the adsorber, methods that replaced the top layer of the adsorbent with a layer of “heavy” adsorbent or support balls were developed and utilized. These methods, however, have a number of disadvantages. First, the heavy adsorbent or support balls add to the regeneration heating load of the adsorber, thus, requiring more power for regeneration. Second, the heavy adsorbent or support balls tend to migrate in the adsorber. To combat the migration of the heavy adsorbent or support balls, interlayer fine wire mesh screen may be used to stop such migration; however, introduction of the interlayer fine wire mesh screen in the adsorber itself presents further issues in that such screen does not fit perfectly flush with the near wall of the adsorber vessel. Thus, a small void area exists between the interlayer fine wire mesh screen and the near wall of the adsorber vessel. Such void areas may be as small as a few centimeters, for example, but such void area may be significant enough for adsorbent to enter these void areas. Once in these void areas, the adsorbent will begin to move and fluidization will occur. In fact, in such cases, the fluidization may cause the void areas to become larger creating further issues with the adsorber efficiency.
U.S. Pat. No. 4,353,716 to Rohde disclosed a process and apparatus for regenerating an adsorber. The disclosed apparatus included a bulkhead located in the zone of the vessel wall for separating the adsorbent arranged in the zone adjacent to the vessel wall from the remaining adsorbent in the vessel. The space between the vessel wall and the bulkhead was, however, filled with adsorbent material and a secondary feeding or discharge means for regenerating gas was also provided. The disclosed design attempted to improve regeneration purge near the vessel wall to ensure an optimum regeneration of the entire adsorbent that thereby reduced the cost of manufacturing and increases operating safety.
The bulkhead arrangement disclosed in U.S. Pat. No. 4,353,716 exhibited serious disadvantages. The small cross-sectional area created by the bulkhead and vessel wall leads to larger voids or empty space for the gas to flow through. Such increased voids can cause flow channeling which may actually enhance fluidization and dusting in the localized region. Thus, there is a need in the art for an improved adsorber process and apparatus design that prevents fluidization of the adsorbent in the adsorber thereby leading to more efficient performance of the adsorbers.